Method and circuit for compensating MOSFET capacitance variations in integrated circuits

ABSTRACT

A method for tracking the MOS oxide thickness by the native threshold voltage of a “native” MOS transistor without channel implantation for the purpose of compensating MOS capacitance variations is achieved. The invention makes use of the fact that in MOS devices the threshold voltage is proportionally correlated to the oxide thickness of said MOS device. The threshold voltage can therefore be used to build a reference voltage V x +V th  which accurately tracks the MOS capacitance variations in integrated circuits. Circuits are achieved to create a frequency reference and a capacitance reference using said method Additionally a method is introduced to create a capacitance reference in integrated circuits using said MOSFET capacitors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/119,924 filed on Apr. 10, 2002 now U.S. Pat. No. 6,774,644.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates generally to integrated circuits requiringaccurate capacitance values. Specifically, the present inventionintroduces a method to use MOS devices as stable capacitors andcomprises a method and circuits used for compensating capacitancevariances of said MOS capacitors in integrated circuits in applicationsas oscillators, frequency references and capacitance references and amethod to build capacitance references in integrated circuits.

(2) Description of the Prior Art

The capacitance of available capacitors in integrated circuits isvarying more than 10% to 25%. Due to said variance of the capacitanceapplications requiring an accurate value of capacitance, as e.g. asuitable frequency reference cannot be implemented efficiently inintegrated circuits. Therefore in integrated circuits said problems aresolved in prior art by trimming e.g. an oscillator using fuses which isexpensive and consuming tester time.

Typical applications where accurate values of capacitance are requiredare e.g. oscillators. Oscillators are circuits for convening dc powerinto a periodic wave-form or signal. Conventional RC oscillatorsadvantageously furnish a low-cost timing source and allow for generationof variable frequencies by changing the resistance R, or capacitance C.Furthermore, conventional RC oscillators avoid advantageously the use ofinductors, which are difficult to fabricate in integrated circuits.

Normally an RC relaxation oscillator needs a precision resistor R andalso a precision capacitor C to achieve an accurate time constant T=R×C.Since both devices vary by 10-25% in integrated circuit fabrication anexpensive trimming scheme or external components must be used for such afrequency reference. FIG. 1 prior art shows as example the principle ofsuch a relaxation oscillator. Said RC oscillator comprises the currentsource 1 with a current ${{Iref} = \frac{Ur}{R}},$a periodical switch 2, a capacitor 3 having the capacitance C, acomparator 4 having as input the voltage U_(c) 21 at the capacitor C 3and the threshold voltage U_(th) 4 and as output the voltage U_(s) 5.Said threshold voltage is proportional to the reference voltage U_(r).U _(th) =k×U _(r)The principle is to charge a capacitor with a current I_(ref)proportional to said reference voltage U_(r). If the resulting voltageat the capacitor C 3 exceeds said threshold voltage U_(th) 4 a pulse ordigital signal is created at the circuit output U_(s) 5. Afterwards saidcapacitor C 3 is discharged/charged with the same current in order toinitiate another switching event after a certain time which is definedby the switch 2. This is repeated continuously and periodically andtherefore creates a constant frequency at the circuit output U_(s) 5.FIG. 2 prior art shows the voltage U_(c) 21 at the capacitor C 3 (shownin FIG. 1 prior art) and FIG. 3 prior art shows the voltage at thecircuit output U_(s) 5.

The frequency f of said oscillator follows the formula$f = {\frac{Iref}{2 \times k \times {Ur} \times C} = \frac{1}{2 \times k \times C \times R}}$It is obvious that the accuracy of the values of the resistor R and thecapacity C have a direct impact to the frequency of said oscillator.Variations in the order of magnitude of 25% are not acceptable for mostapplications.

U.S. Pat. No. (6,020,792 to Nolan et al.) describes a precisionrelaxation oscillator with temperature compensation. The precisionrelaxation oscillator is comprised of an oscillation generator comprisedof a set-reset flip-flop and other components, a first current generatorfor producing a first output current and a second current generator forproducing a second output current. The invention is implemented on asingle, monolithic integrated circuit.

U.S. Pat. No. (5,801,411 to Klughart) discloses an integrated capacitorstructure having substantially reduced temperature and voltagecoefficients including a combination of conventional N-depletion andP-depletion MOS gate capacitors connected in parallel and optimized foruse at low bias voltages, where both the N-depletion and P-depletioncapacitor structures have substantially zero temperature coefficients intheir fully depleted region of operation.

U.S. Pat. No. (5,585,765 to O'Shaughnessy) shows a low power RCoscillator including a low power bias circuit and an RC network. The RCnetwork is used to form a time constant equal to the RC product. The RCoscillator includes a separate oscillator, such as a voltage-controlledoscillator (VCO), and uses the RC time constant to compare with theoscillator-generated period and to adjust the frequency of the overallRC oscillator circuit in accordance with the comparison. The RCoscillator is self-calibrating.

W. M. Sansen et al. (IEEE Journal of Solid State Circuits, Vol. 23, No.3, June 1988) describe a temperature-compensated current reference forCMOS integrated circuits, based upon a MOSFET as current definingelement. So as to minimize the mass production cost, it uses no externalcomponents nor trimming procedures. Comparison with classical currentreference with a resistor as a current defining element shows aconsiderable improvement of the relative tolerances of the current.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a method forcompensating the variations of the capacitance of capacitors inintegrated circuits.

A further object of the present invention is to achieve a frequencyreference with MOS capacitors in integrated circuits.

Another further object is to achieve a capacitance reference usingMOSFET capacitors in integrated circuits.

Another further object of the invention is to achieve a method to createa reference capacitor using an RC oscillator and a MOSFET capacitor inintegrated circuits.

In accordance with the objects of this invention a method for trackingthe MOS oxide thickness by the native threshold voltage V_(th) of a“native” MOS transistor for the purpose of compensating MOS capacitancevariations is achieved. Said method comprises, first, providing a MOStransistor having a native threshold voltage V_(th). The first step isto sense the native threshold voltage Vth of said native MOS transistor,followed by building a suitable reference voltage level by adding by acircuit means another constant voltage V_(x), then charging a MOScapacitor built out of a native MOS transistor up to the reference levelV_(ref)=V_(x)+V_(th) and finally measure a capacitance related value ina suitable configuration for oscillators which depends now no more onthe oxide thickness.

In accordance with the objects of this invention a circuit for afrequency reference in an integrated circuit using a MOSFET as a stablecapacitor is achieved. Said circuit, first, is comprising a constantcurrent source, switching means charging and discharging said MOSFETcapacitor being activated periodically by the output of comparing meanscharging and discharging said MOSFET capacitor. Furthermore said circuitis comprising comparing means having an input and an output wherein theinput is the voltage of said capacitor and a voltage exceeding thethreshold voltage of said MOSFET to influence the frequency and theamplitude of the output of said comparing means and the output is aperiodical pulse. Furthermore the circuit comprises a MOSFET capacitorbeing charged/discharged periodically by said constant current sourceand its voltage level being the input to said comparing means and itsthreshold voltage is used to compensate the variations of capacitance,means to provide a voltage exceeding the threshold voltage of saidMOSFET to influence the frequency and the amplitude of the output ofsaid comparing means, and means to transform the periodical pulse intoan alternating clock signal to drive said switching means and being theoutput of said frequency reference circuit.

In accordance to further objects of the invention a circuit to use aMOSFET capacitor as a reference capacitor in an integrated circuit isachieved. Said circuit, first, is comprising an RC-oscillator circuithaving an input and an output wherein the input is a current source anda signal from a phase detector regulating the capacitance of said RCoscillator and the output is a periodical signal, a frequency referencecircuit having an input and an output wherein the input is a currentsource and the output is a periodical signal and a phase detectorcircuit having an input and an output wherein the input are theperiodical output signals from said RC oscillator and from saidfrequency reference and the output is a signal to regulate thecapacitance of the RC oscillator and the capacitance of the capacitor tobecome a reference capacitor and said capacitor to become a referencecapacitor.

In accordance to further objects of the invention a method for creatinga capacitance reference in integrated circuits is achieved said methodis comprising, first, providing an RC oscillator circuit, a constantcurrent source, a frequency reference, a phase detector and a capacitorto become a reference capacitor. The first step is generating with saidRC oscillator circuit a periodic signal with frequency f₁ followed bygenerating with said frequency reference-circuit a signal with frequencyf₂, then comparing said frequency f₁ and frequency f₂ and regulating thecapacitance of the capacitor of said RC oscillator until f₁ equals f₂,calculate the capacitance of the reference capacitor using an equationand use the calculated capacitance for a reference capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIG. 1 prior art illustrates a block diagram of the principle of an RCoscillator.

FIG. 2 prior art is a time chart of the voltage at the capacitor side ofan RC oscillator.

FIG. 3 prior art is a time chart of the voltage at the output of an RCoscillator.

FIG. 4 shows a principal layout of a MOSFET.

FIG. 5 shows the principal components of a circuit of a frequencyreference using MOSFET capacitors.

FIG. 6 illustrates a block diagram of a circuit of a capacitancereference

FIG. 7 shows a flowchart of the method how to a achieve a referencecapacity using a MOSFET capacitor.

FIG. 8 shows a flowchart illustrating a method how to track MOS oxidethickness by a native threshold voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments disclose methods and novel circuits tocompensate the variations of the capacitance of MOS capacitors and toachieve frequency references and capacity references using MOScapacitors in integrated circuits. The invention avoids the costlytrimming of capacitors in integrated circuits as performed in prior art.

FIG. 4 shows the basic structure of a n-type Metal-Oxide-Semiconductorfield-Effect-Transistor(MOSFET). It consists of a source 45, a drain 44,two highly conducting n-type semiconductor regions which are isolatedfrom the p-type substrate 46 by reversed biased p-n diodes. Between thep-type substrate 46 on one side and the drain 44 and the gate 45 on theother side is a depletion layer 43. A metal(or polycrystalline) gate 41covers the region between source and drain, but is separated from thesemiconductor by the gate oxide 42. The back contact is signified with47. Said gate oxide 42 blocks any carrier flow and has therefore acapacitance. The thickness T_(ox) of the gate oxide is in the order ofmagnitude between about 30 to 150 angstrom. If a bias voltage is appliedto the gate metal, relative to the silicon substrate, in excess of thethreshold voltage then charge carriers are gathered in sufficientconcentration under the gate oxide 42.

It is known by those skilled in art that the “native” threshold voltageV_(th) of MOS devices is proportionally correlated to the variation ofthe oxide thickness T_(ox) of said transistors. A “native” MOS device,which is used in the invention, is actually an MOS device withoutchannel implantation. The implantation normally adjusts the thresholdvoltage V_(th) to the desired value. But without such “correction” saidthreshold voltage V_(th) is directly related to the oxide thickness ofsaid MOS device and furthermore the gate capacitance of said transistoris proportionally related to its oxide thickness T_(ox). Therefore alinear correlation between said “native” threshold voltage and the gatecapacitance of said MOS device exists. This has been proven byspectroscopy techniques measuring the oxide thickness and the gatecapacitance. The invention proposes to use said “native” MOS device,this means MOS devices without any channel implantation, as capacitorsin integrated circuits. The capacitance of the oxide layer of saidnative MOSFET relates to ${C({oxide})} = \frac{a}{T_{ox}}$andthe threshold voltage relates toV _(th) =b+c×T _(ox).

T_(ox) is the oxide thickness, a, b and c are physical constants ordoping values which are assumed to be fix or at least accuratelypredictable. After said threshold voltage V_(th) is sensed it can beused build a reference voltageV _(ref) =V _(x) +V _(th)which accurately tracks the capacitance variations of said MOS device inintegrated circuits such as e.g. oscillators. The value of V_(x) ischosen to achieve a proper compensation, in the preferred embodimentV_(x)=−b, wherein b is the physical constant mentioned above in theequation to define V_(th).

Said reference voltage V_(ref)=V_(x)+V_(th) plays the role of thereference voltage k×U_(r) shown in the prior art FIGS. 1 and 2.Therefore a suitable voltage V_(x) can be defined such that thereference voltage V_(ref) effectively compensates the impact of thevariation of the oxide thickness T_(ox) on the output frequency of suchan oscillator so that the remaining frequency stays independent of saidoxide thickness variations. Any decrease of T_(ox), for example, whichwould increase the capacitance C and decrease the frequency f resultsalso in a decrease of the threshold voltage V_(th) and the relatedreference voltage V_(ref)=V_(x)+Vth and compensates the frequencyvariation caused by the change of the oxide thickness.

FIG. 8 illustrates a method for tracking MOS thickness by said “native”threshold voltage V_(th) of a “native” MOS device for the purpose ofcompensating MOS capacitance variations. The first step 81 shows that anative MOS device without channel implantation has to be provided. Instep 82 the “native” threshold voltage V_(th) is sensed In the next step83 a suitable reference voltage V_(ref)=V_(x)+V_(th) is built by addingthe constant voltage V_(x) to the threshold voltage V_(th). Said MOScapacitor is charged up to said reference level V_(ref)=V_(x)+V_(th) instep 84. Step 85 illustrates that a capacitance related value as e.g.the charging time of said MOS capacitor or the resulting frequency of asuitable configuration as e.g. a feedback configuration or a repeatingconfiguration is measured. Said capacitance related value can be e.g.the charging time or the resulting frequency. The charging time dependson the oxide thickness, since this determines the capacitance, and alsoon the reference voltage. Both influences compensate each other inoptimized conditions.

Another advantage of said invention is that the impact of changes of thesupply voltage Ur to the frequency f is very small. V_(x) is usuallymuch greater than said threshold voltage V_(th). Therefore, if V_(x) isderived from the supply voltage U_(r), the resulting frequency f is infirst order independent of U_(r), since the current source I_(ref) alsotracks any U_(r) variations.

The usage of such a MOS capacitor presented is especially suitable tobuild a frequency reference together with an external precisionresistor. FIG. 5 shows an example of a circuit that can perform saidmethod of tracking MOS oxide thickness by the native threshold voltageof a “native” MOS device for the purpose of compensating MOS capacitancevariations. In said circuit the required comparator and the voltageaddition V_(ref)=V_(x)+V_(th) within the same building block isrealized.

The circuit is comprising two said “native” MOSFET capacitors 51 and 52which are switched by the clock signals Φ1 and Φ2. Said two MOSFETcapacitors as all other transistors of the circuit are “native”transistors, this means without channel implantation. The relatedswitches for the frequency Φ1 are the transistors 53 and 56, theswitches for the frequency Φ2 are the transistors 54 and 15. Said clocksignals are inverted to each other, therefore one of the two saidcapacitors is always connected to zero while the other while the othergets charged during one half time period.

The amplifier 55 is connected through the transistor 6 and the voltagedivider consisting of the resistors R1 8 and R2 9 in a simple feedbackconfiguration. It regulates the voltage at node “A” to the value:${Vx} = {\frac{{Ur} \times \left( {{R2} + {R1}} \right)}{R1}.}$(Therefore V_(x) can be accurately generated. In this context saidtransistor 6 operates as a voltage follower.

Additionally said transistor 6 forms the input of a comparatorcomprising the pair of transistors 10 and 11 and additionally thetransistors 57 and 58 form a current mirror to complete said comparatorbuilt as a differential amplifier. Since the MOSFET capacitors 51 and 52are switched alternatively either transistor 10 or 11 is active at thesame time. Because of the source follower operation of transistor 6 inconnection with the amplifier 55 the voltage at node “B” is exactlyV_(ref)=V_(x)+V_(th). Now if “native” NMOS technology is chosen fortransistors 6, 10, 11 then V_(ref) forms a suitable reference levelwhich is connected to the capacitor input of said comparator.

Therefore said comparator generates a short positive pulse throughtransistor 60 at output node “C” as soon as the capacitor voltage at theinput of said comparator exceeds V_(ref). The resistor 59 represents acurrent source. The toggle flip-flop (T-FF) 12 transforms the positivepulses into alternating clock signals which in turn switches the MOSFETcapacitors 51 and 52. The two inverters 13 and 14 are inverting theclock signals Φ1 and Φ2.

Further the presented invention with said RC oscillator can be used tocreate a capacitor reference. Since the resulting frequency, accordingto the given equations, only depends on U_(r), R and on physicalconstants, an equivalent capacitance can be calculated. This would beindependent of the resistor R and can therefore be fabricated with highaccuracy in an IC manufacturing process. FIG. 6 illustrates how acapacitor reference can be implemented by combining an RC oscillator 61,as shown in FIG. 1 prior art, with the frequency reference 64 shown inFIG. 5 in detail. The output frequency of the RC-oscillator 61 is f₁.The output frequency of the frequency reference 64 is f₂. The capacitorin the RC-oscillator 61 must be of the same type as the capacitor C₀which is to become a reference capacitor and will be trimmed therefore.From the given equation results${f_{1} = \frac{1}{2 \times k \times {C1} \times R}},{f_{2} = {\frac{Ur}{2 \times \left( {{Vx} + {Vth}} \right) \times {Cox} \times R} = {\frac{{Ur} \times {Tox}}{2 \times a \times R \times \left( {{Vx} + b + {c \times {Tox}}} \right)}.}}}$

choosing e.g. V_(x)=−b to compensate the capacitance variations resultsin: $f_{2} = {\frac{Ur}{2 \times R \times a \times c}.}$

In above equations a, b and c are the same physical constants or dopingvalues as mentioned earlier in the equations for C(oxide) and for thethreshold voltage V_(th).

Now f₁ and f₂ can be compared with a “phase detector” PD 63 whichregulates in a feedback loop capacitor C₁ in the RC oscillator 61 suchthat f₁ equals f₂. The regulation is done by switching parts of C₁ “on”and “off” implemented preferably by MOS switches. Said condition “f₁equals f₂” is true If the clock edges of the signals f₁ and f₂ aresynchronized. Any types of capacitors can be used for C₁ and C₀. C₀always, independent of f₁ or f₂ equals C₁ since it is matched in thecircuit.

In the said state of equilibrium of f₁ and f₂ the equations are valid:$\begin{matrix}{\frac{Ur}{2 \times R \times a \times c} = \frac{1}{2 \times k \times R \times C_{1}}} \\{and} \\{C_{1} = {C_{0} = {\frac{a \times c}{{Ur} \times k}.}}}\end{matrix}$

With the help of above mentioned equation the capacitance of thecapacitor C₀ can be calculated and C₀ can be used as a referencecapacitor.

If the same output signal of the “phase detector” 61 at node “A” drivesthe reference capacitor C₀ as well as the capacitor C₁ said referencecapacitor gets very much predictable and accurate. The parameters a, cand k are physical constants or integer numbers, the voltage Ur caneasily be generated accurately in integrated circuits.

FIG. 7 shows using a flowchart a summary of the method to create acapacitor reference. The steps 71 and 72 illustrate that the RCoscillator 61 is generating a frequency f₁ and the frequency reference64 is generating a frequency f₂. Step 73 illustrates that a phasedetector 63 compares said frequencies f₁ and f₂ and regulates thecapacitance of the capacitor of said RC oscillator 61 until f₁ equalsf₂. Both frequency generators and the phase detector are shown in FIG.6. In the next step 74 the capacitance of the reference capacitor iscalculated using the above mentioned equation. The reference capacitorcan be used now having the calculated capacitance.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A circuit to use a MOSFET capacitor as a reference capacitor in anintegrated circuit comprising: an RC-oscillator circuit having an inputand an output wherein the input is a current source and a signal from aphase detector regulating the capacitance of said RC oscillator and theoutput is a periodical signal; a frequency reference circuit having aninput and an output wherein the input is a current source and the outputis a periodical signal and wherein said frequency reference circuit iscomprising: a constant current source; switching means charging anddischarging said MOSFET capacitors being activated periodically by theoutput of comparing means and charging and discharging said MOSFETcapacitor; comparing means having an input and an output wherein theinput is the voltage of said MOSFET capacitor and a voltage exceedingthe threshold voltage of said MOSFET to influence the frequency and theamplitude of the output of said comparing means and the output is aperiodical pulse; a MOSFET capacitor being charged and dischargedperiodically by said constant current source and its voltage level beingthe input to said comparing means and its threshold voltage is used tocompensate the variations of its capacitance; means to provide a voltageexceeding the threshold voltage of said MOSFET to influence thefrequency and the amplitude of the output of said comparing means; andmeans to transform the periodical pulse into an alternating clock signalto drive said switching means and being the output of said frequencyreference circuit; a phase detector circuit having an input and anoutput wherein the input are the periodical output signals from said RCoscillator and from said frequency reference and the output is a signalto regulate the capacitance of the RC oscillator and the capacitance ofa capacitor to become a reference capacitor; and said capacitor tobecome a reference capacitor.
 2. The circuit of claim 1 wherein saidswitching means are transistors.
 3. The circuit of claim 1 wherein saidstable current source is using a reference voltage and an resistorexternal to the integrated circuit.
 4. The circuit of claim 1 whereinsaid comparing means are a pair of transistors.
 5. The circuit of claim1 wherein said MOSFET capacitor is a pair of MOSFETs which arealternating charged and discharged.
 6. The circuit of claim 1 whereinsaid means to provide a voltage exceeding the threshold voltage of saidMOSFET to influence the frequency and the amplitude of the output ofsaid comparing means are a voltage divider and an amplifier having aninput and an output wherein the input is the output of said voltagedivider and a reference voltage and the output is said voltage as inputof said comparing means.
 7. The circuit of claim 1 wherein the saidcapacitor is a pair of MOSFETs.
 8. The circuit of claim 1 wherein aflip-flop circuit and inverters are used to transform the output pulsesof the comparing means into alternating clock signals being the input tosaid switching means forming the output of said frequency referencecircuit.